THE HEATER ON MY 2000 CADILLAC DEVILLE ONLY BLOWS HOT PERIODICALLY. WHEN I START THE CAR I HEAR A CLICKING NOISE AT THE CONTROL PANEL. AFTER THE CAR WARMS UP THE TEMP CAGE FLUCTUATES BETWEEN NORMAL AND A LITTLE ABOVE NORMAL. DO I HAVE A ACTUATOR PROBLEM?

You did mean (TEMP GAUGE) didn't you? If that fluctuates I don't think that's normal for this car. Once warmed up especially if you are in cold enough weather to request heat it should stick to the exact spot all the time IMO.

For now you need to know if coolant is full at the radiator not just the tank you can see thru. If air or some combustion gasses are getting into cooling system the heat transfer will be all screwed up - erratic if slightly low and no heat if very low and this car should have a warning light for low coolant when low enough to cause real engine problems,

Thanks HT for jumping on that. I stopped as there were several models of "DeVille" listed and all showed left and right actuators suggesting temp controls were all dual zone which stopped me. No info on time to do that I have. Under an hour sure seems pretty simple - good.

As you said if cooling system is full ( I focused or the word "erratic" and didn't concentrate on any noise) it's just having a problem holding the requested blend IMO.